title: The determination of the minimal nitrogen excretion in steers and dairy cows and its physiological and practical implications.
abstract: 1. Cattle were maintained by intragastric infusion to see how much nitrogen was excreted on protein-free diets. 2. Minimal N excretion was estimated with two dairy cows in three periods, i.e. when they were non-pregnant and non-lactating, when they were between 117 and 133 d pregnant and when they were between 220 and 233 d pregnant. The minimal N excretion was also estimated on two occasions with two steers when their average live weights were 200 and 350 kg. 3. Average urinary N excretion without protein infusion was 298, 305 and 283 mg/kg metabolic live weight (W0.75) for the non-pregnant cows and for cows during the first and second periods of pregnancy respectively; total N excretion including the faecal N was 340, 329 and 319 g/kg W0.75. 4. For steers the urinary N values were 403 and 295 mg/kg W0.75 at 200 and 350 kg live weight respectively and total N excretion including faecal N was 408 and 320 mg/kg W0.75. 5. Urinary excretion of creatinine was the same for animals given casein via the abomasum as a source of protein or given no protein with mean values for the cows of 13.6 and 14.9 g/d for the first and second stages of pregnancy respectively. Mean values for the steers were 6.5 and 7.6 g creatinine/d at 200 and 350 kg live weight respectively. 6. It is suggested that the so-called metabolic faecal N in ruminants, estimated with N-free diets, is mainly, endogenous N derived from tissue breakdown of protein but incorporated in microbial debris and excreted in the faeces.
mesh_terms: D000818:Animals; D001835:Body Weight; D002417:Cattle / Q000502:physiology*; D003404:Creatinine / Q000652:urine; D005243:Feces / Q000032:analysis*; D005260:Female; D008297:Male; D009584:Nitrogen / Q000032:analysis* / Q000652:urine; D010289:Parenteral Nutrition, Total; D011247:Pregnancy; D011506:Proteins / Q000008:administration & dosage; D014508:Urea / Q000652:urine
chemical_list: D011506:Proteins; D014508:Urea; D003404:Creatinine; D009584:Nitrogen